Magazine article Science News

The Moon's Tug Stretches out the Day

Magazine article Science News

The Moon's Tug Stretches out the Day

Article excerpt

If the hours seem to streak by and each day ends too quickly, stop a moment and consider the tempo of time during the Proterozoic era, 900 million years ago. Earth revolved 30 percent faster back then, and the day lasted only 18.2 hours, according to a team of scientists studying ancient rock deposits that record lunar tides.

Using theories of celestial mechanics, physicists long ago surmised that the day should be growing longer because tides raised principally by the moon put a brake on Earth's rotation. By robbing Earth of momentum, the moon edges farther away, thus increasing its velocity through space.

Modern measurements have verified part of the theory by showing that the moon is receding from Earth at 3.8 centimeters per year. But proving that the Earth has slowed down requires records from the distant past.

Charles P. Sonett of the University of Arizona in Tucson and his colleagues gleaned the history of Earth's spin from sedimentary stones known as tidal rhythmites. These rare rocks formed along prehistoric shorelines, where tides deposited alternating bands of dark- and light-colored silt and sand.

The rhythmites display layers of varying thickness, reflecting the high spring tides and low neap tides that mark the lunar month. By analyzing how the tidal cycles vary in thickness with the seasons, the scientists could count the number of lunar months per year. This enabled them to calculate how much momentum the moon had stolen from Earth. From that, they could figure ancient Earth's rate of rotation.

Sonett's group examined four sets of tidal rhythmites, from Indiana, Alabama, Australia, and Utah, with ages ranging from 305 million to 900 million years. When the oldest rocks formed, each year had 481 days, the scientists report in the July 5 Science.

Although simple in theory, the study of these deposits is complicated; ancient storms and other factors have erased some of the tidal layers. …

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